This invention relates to forming thermoplastic interlayer sheeting and more particularly to a method for forming discrete, shaped blanks of plasticized polyvinyl butyral suitable for laminated safety glass windshields.
Thin sheet formed of plasticized polyvinyl butyral is well known as an interlayer in laminated safety glass finding application in windows such as penetration-resistant automobile windshields. A very well known problem in view of many patents dealing with it since the first in the United States issued in 1952, is the performance deficiency in the interlayer when used in a specially curved and angled windshield. More specifically, sheet in such applications has traditionally been shaped in association with glass laminating downstream of sheet forming by stretching into a shape to match the desired windshield configuration. Special problems arise when the interlayer has a colored band along one margin which is intended to reduce glare from the sun when in place in the windshield. The band is usually graduated in color intensity with the greatest color along the upper peripheral portion of the windshield which then gradually diminishes to an almost imperceptible cutoff line at the lower edge of the band. When such gradient band windshield has horizontal and vertical curvature, as is usual with modern wrap around auto windshields, it has been necessary to stretch each sheet section to an arcuate shape before associating it with the glass so that after lamination the cut-off line of the band is parallel to the upper edge of the windshield. This amount of initial stretching compensates for the initial lack of curvature of the color band which if unstretched would produce a cut-off line not parallel to the upper edge of a curved, wrap-around windshield.
As well delineated in the prior art, for example col. 2, lines 3-28 of U.S. Pat. No. 4,244,997, stretching of sheet after its formation adversely affects its subsequent performance in a windshield. More specifically, differential stretching inherently results in thickness reduction and a buildup of strain unless relieved. Non-uniform thickness translates to variable impact resistance in the windshield, such resistance being lower in the area of reduced thickness. Uneven strain results in variable shrink-back and reduction in peripheral size of the sheet during handling before trapping it in place between layers during lamination. When this occurs the shaped blank must be discarded if it reverts to a smaller size than the glass sheets with which it is being laminated. Reheating sheet previously conventionally formed with a rough surface to facilitate deairing during lamination can prematurely undesirably reduce such roughness rendering it more susceptible to stack sticking in storage and less effective in allowing air to escape during lamination. Further, reheating and stretching sheet previously having predetermined levels of moisture carefully incorporated therein to help control its level of adhesion to adjoining glass layers can cause such moisture to flash out of the sheet. It would be desirable to provide a system for forming interlayer windshield blanks reducing or eliminating the foregoing plethora of shortcomings.